1. Field of the Invention
The present invention relates to an optical encoder that detects a position, and more particularly, to an optical encoder that detects position information relating to a focus lens and a zoom lens included in a lens apparatus such as a still camera or a video camera.
2. Description of the Related Art
Conventionally, an encoder is used to detect a position of a machine tool or a factory automation (FA) apparatus. A position detection system for the encoder is roughly classified into an incremental system for measuring a relative movement distance and an absolute system for detecting an absolute position. The encoder using the incremental system has the advantage that its configuration may be simple while having disadvantages in that position information is lost when power thereto is turned off, and errors are accumulated by external noise. On the other hand, the encoder using the absolute system generally has the advantages that it is high in accuracy because no errors are accumulated and movement to a home position is not required even when power thereto is turned off.
The encoder using the absolute system for detecting an absolute position has a configuration illustrated in FIG. 14, for example, as discussed in Japanese Patent Application Laid-Open No. 2009-198318. According to this example, more information can be obtained from a small number of scale tracks by forming a plurality of modulations having different periods in a scale pattern on one track.
When analysis is performed based on a conventional example, an image received on a sensor surface via a scale including a plurality of modulation periods includes unnecessary spatial frequency components generated by interference between diffracted light beams and unnecessary spatial frequency components included in a transmittance distribution or a reflectance distribution in the scale. An error from an ideal sine wave occurs due to this effect. As a result, the error becomes a position detection error when the sine wave is converted into a position signal by an arc tangent operation.